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United States Patent |
5,200,635
|
Kaga
,   et al.
|
April 6, 1993
|
Semiconductor device having a low-resistivity planar wiring structure
Abstract
The present invention concerns a semiconductor device having a
low-resistivity wiring structure. Wirings formed directly on a hill and
valley structure result in a thin portion and, in an extreme case, a
disconnected portion. This increases the resistivity of wirings on the
hill and valley structure and lowers the reliability of the connection. In
a case where the wirings are data lines of a memory, with an increased
effective length, the resistance and the parasitic capacitance of the data
line is greater. The above mentioned problems have been solved by wirings
which comprise at least two layers of conductive film including a first
conductive film as a lower layer and a second conductive film as an upper
layer, and the first conductive layer has a surface moderating or
planarizing the hills and valleys in the underlying material.
Inventors:
|
Kaga; Toru (Urawa, JP);
Kimura; Shinichiro (Kunitachi, JP);
Kimura; Katsutaka (Akishima, JP);
Nakagome; Yoshinobu (Hachiouji, JP);
Hisamoto; Digh (Kokubunji, JP);
Kawamoto; Yoshifumi (Tsukui, JP);
Takeda; Eiji (Koganei, JP);
Iijima; Shimpei (Akishima, JP);
Kure; Tokuo (Nishitama, JP);
Nishida; Takashi (Taitou, JP)
|
Assignee:
|
Hitachi, Ltd. (Tokyo, JP)
|
Appl. No.:
|
686757 |
Filed:
|
April 17, 1991 |
Foreign Application Priority Data
| Apr 17, 1990[JP] | 2-099439 |
| May 09, 1990[JP] | 2-117573 |
| May 14, 1990[JP] | 2-121117 |
Current U.S. Class: |
257/306; 257/308; 257/384; 257/E27.089; 257/E29.122; 257/E29.146 |
Intern'l Class: |
H01L 029/68; H01L 023/48; H01L 029/04 |
Field of Search: |
357/23.6,71,59 K,59 I,71 S
437/200
|
References Cited
U.S. Patent Documents
4674176 | Jun., 1987 | Tuckerman | 357/71.
|
4974055 | Nov., 1990 | Haskell | 357/71.
|
4994893 | Feb., 1991 | Ozaki et al. | 357/23.
|
5012309 | Apr., 1991 | Nakayama | 357/23.
|
5045899 | Sep., 1991 | Arimoto | 357/23.
|
Other References
IEEE Transactions on Electron Devices, vol. 38, No. 2, Feb. 1991, pp.
255-261.
1990 Symposium on VLSI Technology (IEEE), pp. 13-14.
|
Primary Examiner: Hille; Rolf
Assistant Examiner: Limanek; Robert
Attorney, Agent or Firm: Antonelli, Terry, Stout & Kraus
Parent Case Text
The present application is a continuation-in-part of copending application
Ser. No. 475,148 filed Feb. 5, 1990 now U.S. Pat. No. 5,140,389 issued
Aug. 18, 1992; which is a continuation-in-part of Ser. No. 287,881 filed
Dec. 21, 1988, which issued to U.S. Pat. No. 4,970,564 on Nov. 13, 1990;
each of which is incorporated by reference herein in its entirety.
Claims
What is claimed is:
1. In a dynamic random access memory (DRAM), formed at a main surface of a
substrate, including word and data lines and plural memory cells each of
which includes a transistor having an active region and storage
capacitance, the combination wherein:
each data line has a substantially orthogonally overlapping arrangement
with said word lines, with respect to a plan view thereof;
each said active region is disposed, with respect to said plan view, at an
angle direction which is substantially different from that of the
corresponding ones of said word and data lines;
the data lines are disposed as lower level conductive layers than that for
forming charge storage electrodes of said memory cells, said storage
capacitance is characterized as including a storage capacitance portion
thereof that is provided by a side wall which is directioned substantially
perpendicular to that of the main surface of said substrate; and
the data lines are higher level conductive layers than said word lines with
respect to the main surface of said substrate, said data lines are formed
insulatedly above said word lines and over further coductive layers
disposed in spacings between respective word lines.
2. A dynamic random access memory according to claim 1, wherein said
further conductive layers are formed of polycrystalline silicon material.
3. A dynamic random access memory according to claim 2, wherein said
transistor is an MOS transistor, said MOS transistor has a gate formed by
a corresponding one of said word lines.
4. A dynamic random access memory according to claim 3, wherein said data
lines have substantially planar surfaces and are comprised of a stacked
layer arrangement of two different conductive films.
5. A dynamic random access memory according to claim 1, wherein the stacked
layer arrangement includes a metal layer or a metal silicide layer
laminated on a silicon-containing conductor layer.
Description
BACKGROUND OF THE INVENTION
The present invention concerns a semiconductor device and fabrication
method thereof and, more in particular, it relates to a semiconductor
device having a low-resistivity wiring structure and a fabrication method
thereof.
When wirings are formed on a hill and valley structure in existent
semiconductor devices, an insulator has at first been deposited on the
hill and valley structure to planarize the surface and wiring have been
formed on the insulator as described in Japanese Patent Laying Open Hei
1-140644.
If wirings are formed directly on the hill and valley structure, it
sometimes result in a portion lacking in a predetermined thickness and, in
an extreme case, leads to disconnection, but wirings formed by the method
described above can satisfy the required thickness and causes no
disconnection.
SUMMARY OF THE INVENTION
In the prior art described above, however, the depth of contact holes
formed in the insulator differs depending on the kind of the electrodes of
elements, owing to the planarization with the insulator, which results in
a problem in the deterioration of the fabrication accuracy for the contact
holes and in view of the reliability of the conduction at the contact
portions.
That is, owing to the planarization with the insulator, the depth of the
contact holes for connection between the wirings and the electrodes below
the insulator differs depending on the kind of the electrodes of the
element. In particular, the depth of the contact hole to a semiconductor
substrate is increased to bring about a problem of making it difficult to
obtain a high reliability in the conduction of contact upon usual
formation of a wiring layer such as by sputtering.
Further, there has been a problem, also in the fabrication of contact holes
of different depth, that the etching rate to the insulator has to be
increased extremely as compared with that for the material under the
insulator.
The reliability for the connection of a deep contact hole may be increased
by a method of selectively forming tungsten or the like to the contact
hole. However, since wirings across the hill and valley portion with no
contact hole can not be planarized only with such a means, the resistivity
is increased.
Further, a combined use of the planarization with the insulator and the
selective formation of tungsten to the contact holes results in a problem
that planarization of wirings cannot be attained to all of the holes since
the depth of the contact holes differs.
The present invention provides:
a semiconductor device comprising a semiconductor substrate, a
semiconductor element disposed on the semiconductor substrate and wirings
disposed on a structure having hills and valleys in an underlying material
and connected with the semiconductor element, wherein the wirings comprise
at least two layers of conductor films having a first conductor film as a
lower layer and a second conductor film as an upper layer, in which the
first conductor film has a surface moderating or planarizing the hills and
valleys in the underlying material,
a semiconductor device wherein the specific resistivity of the second
conductor film is lower than the specific resistivity of the first
conductor film,
a semiconductor device, wherein the first conductor film contains at least
one of polycrystalline silicon, W and TiN,
a semiconductor device wherein the second conductor film contains at least
one of polycrystalline silicon, W, Al, Cu, Mo, Ti, Ta, WSi.sub.2,
MoSi.sub.2, TiSi.sub.2, TiW and TiN,
a semiconductor device comprising a semiconductor substrate, a capacitor
disposed on the semiconductor substrate and a transistor disposed on the
semiconductor substrate, the capacitor and the transistor constituting a
memory cell in a dynamic random access memory, and further comprising data
lines connected with a first electrode of the transistor, wherein
the data line comprises at least two layers of conductor films having a
first conductor film as a lower layer and a second conductor film as an
upper layer, and the first conductor film has a surface moderating or
planarizing hills and valleys in the underlying material,
a semiconductor device wherein the specific resistivity of the second
conductor film is lower than the specific resistivity of the first
conductor film,
a method of fabricating a semiconductor device, which comprises depositing
a first conductor film on a hill and valley structure, etching the first
conductor film to render the surface thereof as a surface moderating or
planarizing hills and valleys and depositing a second conductor film on
the first conductor film to form a wiring pattern,
a method of fabricating a semiconductor device wherein the first conductor
film contains a polycrystalline silicon and a step of selectively
depositing the second conductor film to form a wiring pattern is included,
and
a method of manufacturing a semiconductor device wherein the second
conductor film is a metal that can be silicided and a step of forming a
wiring pattern by steps of siliciding a portion of the metal layer present
on the first conductor film and removing an unreacted portion of the metal
layer is included.
The first conductor film as the lower layer is formed for improving the
reliability of contact connection and for planarizing and moderating the
hills and valleys in the underlying material.
The second conductor film as the upper layer is formed for lowering the
wiring resistivity.
The wiring material used for the first conductor film is preferably made of
a material having a resistivity higher than that of the second conductor
film as the upper layer but having a good step coverage in the contact
hole or on the hill and valley structure.
The material having the good step coverage usable herein includes
polycrystalline Si formed by a chemical vapor deposition (CVD) process and
doping a dopant such as P or B by ion injection or heat or thermal
diffusion into the polycrystalline Si, or polycrystalline SiO or amorphous
Si applied with doping simultaneously with CVD.
W, TiN and the like may also be used.
For planarizing the surface of the first conductor film, isotropic or
anisotropic dry etching may be used, use of isotropic etching being
preferred, because a step on one side of the step portion can be made more
moderate and the resistivity of the wirings thereon can be lowered.
As the conductive material for the second conductor film as the upper
layer, there can be used, for example, W, Al, Cu, Mo, Ti, Ta, WSi.sub.2,
MoSi.sub.2, TiSi.sub.2, TiW and TiN.
Polycrystalline Si with a lower specific resistivity than that of the
polycrystalline Si of the first conductor film may also be used.
For the deposition of the second conductor film, either of CVD or
sputtering process may be used since the surface of the underlying
material is planarized and the step is moderated.
Further, the wirings may also be formed by planarizing or moderating the
hill and valley structure by using polycrystalline Si for the first
conductor film, then forming a wiring pattern to the polycrystalline Si,
forming thereover a layer of a metal that can be silicided as the second
conductor film, for example, a layer of Ti, W, Mo, Ta, Pt and Co,
siliciding the layer and, subsequently, removing the layer of unreacted
metal.
The first conductor film as the lower layer of the wirings is used for
improving the reliability of the contact connection and for planarizing or
moderating the steps in the hill and valley structure.
Accordingly, although it is necessary that the material of the first
conductor film should partially remain in the valleys of the underlying
material, it may not necessarily remain on the hills so long as the second
conductor film as the upper layer always remains on the hills.
Accordingly, in a case of using polycrystalline Si, for example, for the
first conductor film, planarizing etching may be applied till
polycrystalline Si on the hills is eliminated and WSi.sub.2 may be
deposited thereover, or polycrystalline may be deposited again after
planarization and WSi.sub.2 may be deposited further thereover.
The second conductor film as the upper layer is formed for lowering the
wiring resistivity.
The present invention further provides:
a semiconductor memory device comprising a semiconductor substrate, a
conductor disposed on the semiconductor substrate, an insulator disposed
on the conductor and a buried film disposed between wordlines, the surface
of the insulator disposed on the wordlines and the surface of the buried
film disposed between the wordlines being substantially at an identical
height and planar, and further comprises substantially planar data lines
on the buried film and the insulator,
a semiconductor memory device wherein a plate electrode is present on the
data line,
a semiconductor memory device, wherein the buried film is an insulator,
a semiconductor memory device, wherein the buried film is a conductor,
a semiconductor memory device wherein the plate electrode and the wiring
conductor are isolated by insulation at an equal pitch with that for the
memory cells, and further
a method of fabricating a semiconductor memory device comprising a first
step of forming wordlines by fabricating a first conductor disposed on a
substrate and disposing a first insulator on the wordlines, a second step
of forming a side wall spacer comprising a second insulator on the side
walls of the wordlines, a third step of disposing the surface of the
insulator disposed between the wordlines and surface of the wordlines such
that they are substantially at an identical height and planar by burying
the gap between the wordlines with a third insulator, a fourth step of
etching a desired portion of the third insulator to expose a desired
portion of the substrate and a fifth step of depositing a second conductor
and a fourth insulator and forming datalines by anisotropically
fabricating both of them, further,
a method of manufacturing a semiconductor memory device wherein the first
insulator and the second insulator comprise SiO.sub.2 and the third
insulator comprises Si.sub.3 N.sub.4, further
a method of manufacturing a semiconductor memory device wherein the fourth
insulator comprises SiO.sub.2,
a method of manufacturing a semiconductor memory device wherein the first
insulator and the second insulator comprise Si.sub.3 N.sub.4 and the third
insulator comprises SiO.sub.2,
a method of manufacturing a semiconductor memory device wherein the fourth
insulator comprises Si.sub.3 N.sub.4,
a method of manufacturing a semiconductor memory device, wherein the fourth
step is followed by a step of burying a third conductor in a hole formed
by the etching of the third insulator and the step of embedding the third
insulator is followed by the fifth step, further
a method of manufacturing a semiconductor memory device, wherein the step
of burying the third conductor is followed by a step of depositing a fifth
insulator on the third conductor and a step of etching a desired portion
of the fifth insulator thereby exposing a desired portion of the third
conductor and the step is followed by the fifth step in which the second
conductor is formed at least on a portion where the third conductor is
exposed,
a method of fabricating a semiconductor memory device comprising a step of
forming wordlines of a desired pattern, a step of forming datalines of a
desired pattern, a step of forming an insulator for protection film, a
step of etching to remove the insulator for the protection film in a
capacitor forming region, a step of forming a storage electrode of a
capacitor into a desired pattern, a step of forming a capacitor insulator,
a step of forming a plate electrode, a step of forming a contact hole to
the insulator for the protection film outside of a memory cell region, a
step of depositing a wiring conductor and a step of forming the wiring
conductor into a desired pattern and, further
a method of fabricating a semiconductor memory device, wherein the step of
forming the wiring conductor into a predetermined pattern is followed by
fabrication for the plate electrode into a pattern identical with the
desired pattern.
The method of fabricating the semiconductor memory device described above
can be carried out, for example, by substantially planarizing gaps between
the wordlines in a memory cell region by using an etching back method (a
method of depositing a CVD insulator and then applying etching
substantially for the thickness of the film deposited, by which the space
is filled and the underlying structure can be planarized substantially if
the space is narrow and planarizing the data line.
In the method of fabricating the semiconductor memory device a memory cell
capacitor is formed, for example, in a stage of the wiring forming step.
That is, by fabricating the plate electrode of the capacitor and depositing
and fabricating a wiring conductor substantially thereafter, an interlayer
insulator already present between the plate electrode and the wiring layer
in a memory cell region is saved to attain the reduction of the step by so
much as the film thickness.
The effective length of the datalines can be decreased by planarizing the
datalines and, accordingly, effective resistance value and parasitic
capacitance value of the datalines can be reduced.
On the other hand, in the memory cell region by forming a wiring layer
directly on the plate electrode, there can be obtained an effect of
reducing the step present so far in the memory cell by so much as the film
thickness of the insulator.
Further, the present invention provides:
a semiconductor device having a wiring structure comprising a plurality of
first wirings respectively disposed at a periodic repetition rate (or
predetermined pitch) on a semiconductor substrate and second wirings
respectively disposed at a periodic repetition rate (or predetermined
pitch) on the first wirings by way of an insulator so as to cross the
first wirings with respect to a plan view thereof, wherein the surface of
the second wirings is substantially planar, the second wirings comprise
two layers of conductor films of different materials,
a semiconductor device, wherein the second wirings have a structure in
which a metal layer or a metal silicide layer is laminated on a silicon
containing conductor layer,
a semiconductor device, wherein the silicon-containing conductor layer is
formed within a groove defined by first wirings adjacent to each other and
has a flat surface,
a semiconductor device wherein the silicon-containing conductor layer
comprises polycrystalline Si introduced with doping impurities,
a semiconductor device wherein the first wirings are wordlines and the
second wirings are data lines,
a semiconductor device, wherein the semiconductor device is a dynamic type
memory LSI having a combination of a MOS transistor and a capacitor as a
minimum cell unit (or memory cell), the wordlines as the gate electrode
wirings for the MOS transistor are the first wirings and the data lines
connected to a first electrode of the MOS transistor are constituted with
the second wirings, and a storage electrode of a capacitor is connected to
the second electrode of the MOS transistor,
a method of fabricating a semiconductor device, which comprises:
a step of forming first wirings as a gate electrode (wordline) of a MOS
transistor,
a step of covering the exposed portion of the first wirings with an
insulator,
a step of forming source and drain regions,
a step of exposing a portion of a semiconductor substrate surface in the
source region,
a step of depositing, while introducing impurities, a first Si film having
a thickness at least greater than one-half of a gap between adjacent
wordlines,
a step of entirely etching back the surface of the deposited first Si film
thereby planarizing the surface,
a step of depositing to laminate a metal or metal silicide,
a step of applying a patterning fabrication to a laminate film of the first
Si film and the metal or metal silicide thereby forming second wirings
constituting the data lines,
a step of covering the exposed portion of the second wirings with an
insulator,
a step of forming a first organic material over the entire surface,
a step of selectively removing a predetermined region of the first organic
material thereby forming a hole,
a step of exposing a portion of the semiconductor substrate surface in the
hole,
a step of depositing a second Si film,
a step of forming a second organic material over the entire surface,
a step of etching back the entire surface of the second organic material
thereby exposing the surface of the second Si film formed on the surface
of the first organic material,
a step of selectively removing the second Si film exposed on the first
organic material,
a step of entirely removing the first and the second organic materials
selectively thereby forming a shield comprising the second Si film as a
storage electrode of a capacitor,
a step of forming a capacitor insulator, and
a step of forming the other electrode of the capacitor,
a method of fabricating a semiconductor device, wherein the organic
material is an organic material capable of spin-coating on a substrate
such as a polyimide resin, photoresist or EB resist,
a method of manufacturing a semiconductor device, wherein the second Si
film is an Si film formed by introducing impurities at the same time with
the formation of the film,
a method of manufacturing a semiconductor device, wherein the second Si
film is an Si film introduced with impurities by means of ion implantation
into Si formed in an amorphous state,
a method of fabricating a semiconductor device, wherein a portion of the
semiconductor substrate surface is exposed prior to the formation of the
first organic material over the entire surface, the first organic material
is subsequently formed over the entire surface and then the first organic
material above the region in which the surface of the semiconductor
substrate is exposed is removed to form a hole and further
a method of fabricating a semiconductor device, wherein polycrystalline Si
is formed on the pattern of the organic material formed by photoetching.
In the semiconductor device described above, when the second wirings
disposed across the first wirings are constituted with a laminate film of
the polycrystalline Si and the metal silicide, the polycrystalline Si in
the lower layer is formed relatively thick so as to fill recesses formed
between the first wirings and etched back to planarize the surface, and
then the metal silicide is deposited to form a deposition film.
In the fabrication method of the semiconductor device described above, a
pattern of the organic material is formed in the predetermined region, the
underlying insulation layer is selectively eliminated by using the organic
material as a mask for the etching to expose the conductor or the
semiconductor and then the polycrystalline Si is formed in this state over
the entire surface without removing the organic material.
Since the etching back planarization of the polycrystalline Si situated in
the lower layer of the second wirings by can film the gaps formed between
the first wirings and planarize the surface thereof, the metal silicide
laminated to form as a substantial wiring layer can be formed into a
planar surface.
As a result, since the metal silicide can be formed on the planar
polycrystalline Si, the substantial wiring length can be shortened to
lower the wiring resistivity according to the present invention, as
compared with a case of not filling the grooves with the polycrystalline
Si, in which the metal silicide is extended along the step formed by the
first wirings and, accordingly, the wiring length of the metal silicide
itself is increased and the resistivity can not be lowered.
Further, the organic material used for forming the capacitor portion can be
formed or fabricated free from the effect of contaminations with the metal
silicide present on the surface of the substrate.
In addition, since the organic material can be etched in the fabrication at
a selectivity ratio which is infinitely greater as compared with inorganic
materials used so far in semiconductor fabrication steps such as silicon
dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) films,
polycrystalline Si or Si substrates, even if such inorganic materials are
present in the underlying material and the surface of the inorganic
materials is exposed to a portion of the etching region in the course of
the etching fabrication, only the organic material can be etched to
eliminate selectively without etching such inorganic materials at all.
Further, it has been found that no plastic deformation is caused to the
organic material itself, if heating and temperature elevation are applied
in an atmosphere substantially free from oxygen.
Polycrystalline Si can be formed over the entire surface at a temperature
of about 600.degree. C. in a state where the organic material applied with
patterning fabrication due to the effect of the organic material.
Further, in the prior art, the organic material has been used as a
fabrication mask for the material underlying as typically represented by a
photoresist applied with patterning by photolithography or an electron
beam (EB) resist, and the organic material has been removed after the
finishing of the fabrication to the underlying material.
It is one of the features of the present invention not to remove the
organic material used as the mask for the fabrication of the underlying
material but leave the organic material as it is and utilize it as a
supporting base for forming inorganic materials such as polycrystalline
Si.
A first object of the present invention is to provide a semiconductor
device and a fabrication method therefor, capable of attaining two
advantageous effects together, that is, lowering the resistivity of
wirings on a hill and valley structure and improving the reliability of a
contact connection.
The technique as described in U.S. Pat. No. 4970564 has succeeded in
increasing the surface area of a storage electrode by using a structure in
which data lines are laminated just above the wordlines.
However, as shown in a cross sectional view of FIG. 1 for the lamination
capacitance type cell, since data lines 28 are formed conforming the
surface of the hill and valley structure of the wordlines 23, the
effective length of the datalines is increased to result in a problem of
increasing the wiring resistivity of the data lines.
Further, since the side area of the data lines is remarkably increased,
parasitic capacitance on the sides of the data lines formed between the
side region and other electrode is remarkably increased.
Increase of the resistivity and the parasitic capacitance of the data lines
results in a problem of lowering the signal reading speed or increasing
noises.
Furthermore, since the technique disclosed above uses a structure in which
the datalines and the storage electrode and, further, the plate electrode
are laminated on the wordlines, there is also a problem that a step
between the memory cell region and the peripheral circuit region is
increased.
A second object of the present invention is to provide a semiconductor
memory device and a fabrication method therefor capable of decreasing the
effective length of the datalines and reducing the resistivity and the
parasitic capacitance of the datalines.
A third object of the present invention is to provide a semiconductor
memory device and a fabrication method therefor capable of reducing the
step.
Along with the increase in the integration degree of LSI and the
micro-miniaturization of individual elements used therefor, metal
silicides as low-resistivity material have now been used more and more as
the datalines, particularly, in order to suppress the increase of the
resistivity of the datalines.
FIGS. 2(a)-2(h) show a schematic cross sectional views of an example of
fabrication steps of a memory cell in a case of using a metal silicide for
the datalines.
After forming a thick SiO.sub.2 212 as an isolation region on a Si
substrate 211, an SiO.sub.2 213 as a gate insulator for a MOS transistor
is formed and, subsequently, a gate electrode 214 as word lines is formed.
Thus, data lines comprising a laminate film of a polycrystalline Si 217 and
a metal silicide 218 are formed, an exposed portion of the data lines is
covered with an SiO.sub.2 219, then SiO.sub.2 on the surface of the Si
substrate is removed from a predetermined region in which a capacitor is
to be formed, and a polycrystalline Si 220 is selectively grown from the
exposed surface of the Si substrate (FIG. 2(a)).
An Si nitride film 221 and an SiO.sub.2 222 are laminated to form (FIG.
2(b)).
The SiO.sub.2 222 and the Si nitride film 221 are removed from a
predetermined region forming a capacitor successively by means of
lithography or anisotropic dry etching (FIG. 2(c)).
A polycrystalline Si 224 is formed (FIG. 2(d)).
An organic material 225, for example, a photoresist is spin coated over the
entire surface and etched back entirely by dry etching to bury the organic
material only in the valley, while leaving the polycrystalline Si 224 to
be exposed on the surface of the hill (FIG. 2(e)).
The polycrystalline Si 224 exposed on the surface of the hill is
selectively eliminated (FIG. 2(f)).
The organic material burying the valley is selectively eliminated to expose
the polycrystalline Si 224 as a storage electrode of a capacitor (FIG.
2(g)).
A capacitor insulator 226 (extremely thin) and a capacitor electrode 227
are formed (FIG. 2(h)).
A memory cell can be constituted by way of successive steps as described
above, and wirings are formed in subsequent steps to fabricate into an
LSI.
In the technique as described above, it is intended to lower the
resistivity by using a laminate structure of the polycrystalline Si and
the metal silicide for the data lines but, if it is attempted to increase
the integration degree, the resistivity of the data lines is increased
relatively even by using such a structure, to result in a problem of the
reducing the performance as LSI.
Further, since the metal silicide is used for the data lines, the metal
silicide itself gives contaminations over the entire surface of the Si
substrate which causes a remarkable reduction in the selectivity upon
selective growth of Si conducted in the subsequent step to bring about a
problem of reducing the yield.
A fourth object of the present invention is to provide a semiconductor
device having a wiring structure intended for lowering the resistivity of
wirings formed across the step.
A fifth object of the present invention is to provide a method of
fabricating a semiconductor device capable of avoiding the problem that
the selectivity is remarkably reduced when Si is caused to grow
selectively on a substrate contaminated with the metal silicide, and
capable of forming a d-RAM memory cell with simplified fabrication steps.
These and other objects and many of the attendant advantages of this
invention will be readily appreciated as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view for illustrating a subject to be dissolved
by the present invention;
FIGS. 2(a)-2(h) are cross sectional view of fabrication steps for
illustrating the subject to be discussed by the present invention;
FIG. 3 is a cross sectional view illustrating a constitution of a first
embodiment according to the present invention;
FIG. 4 is a cross sectional view for illustrating the effect of the first
embodiment according to the present invention;
FIG. 5 is a cross sectional view illustrating a constitution of a second
embodiment according to the present invention;
FIG. 6 is a cross sectional view for illustrating the effect of the second
embodiment according to the present invention;
FIG. 7(a) is a cross sectional view of a peripheral circuit region in a
semiconductor memory device as a fifth embodiment of the present
invention,
FIG. 7(b) and FIG. 7(c) are, respectively, cross sectional views of a
memory cell region thereof; and
FIG. 7(d) is a view showing an arrangement thereof;
FIGS. 8(a)-8(l) are cross sectional views of an element illustrating
fabrication steps of a semiconductor memory device as a fifth embodiment
according to the present invention shown in FIG. 7;
FIGS. 9(a)-9(c) are cross sectional views of an element for illustrating
different fabrication steps;
FIGS. 10(a) and 10(b) are plan views showing a layout for a semiconductor
memory device as the fifth embodiment according to the present invention
shown in FIG. 7;
FIGS. 11(a) and 11(b) are plan views illustrating a different layout
thereof;
FIGS. 12(a)-12(d) and 13(a)14 13(d) are, respectively, plan views for the
improved layouts as the fifth embodiment according to the present
invention;
FIGS. 14(a) and 14(b) are cross sectional views for illustrating the
problems discussed hereinabove by the present inventors;
FIGS. 15(a)-15(e) are cross sectional views of successive fabrication steps
showing the sixth embodiment according to the present invention;
FIG. 16 is a planar layout view of a memory cell for illustrating FIG. 17
showing a seventh embodiment according to the present invention;
FIGS. 17(a)-17(m) are cross sectional views of successive fabrication of
steps showing the seventh embodiment according to the present invention;
FIGS. 18(a)-18(d) are cross sectional views of successive fabrication steps
showing an eighth embodiment according to the present invention;
FIGS. 19(a)-19(e) are cross sectional views of successive fabrication steps
showing a ninth embodiment according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Descriptions will now be made with reference to the drawings but it should
be noted that impurity regions are omitted or characteristic portions are
emphasized, sometimes, in the drawings.
Embodiment 1
The first embodiment of the present invention will now be described
referring to FIG. 3.
In FIGS. 3 through 6, are depicted a silicon substrate 101, an isolation
region 102, a gate insulator 102', wordline 103, an opening 105, 203, an
SiO.sub.2 film 106, 106', a first conductor film 107 as a buried
conductor, a second conductor film 108, polycrystalline Si 202, 301, 402,
WSi.sub.2 302, 403, a gate polycrystalline Si 404 and an etching residue
405, respectively.
As shown in FIG. 3, an isolation region, for example, a silicon oxide film
(hereinafter simply referred to as SiO.sub.2 film) 102 for electrically
isolating an element is at first formed to a thickness of 450 nm on a
semiconductor substrate 101 of a desired conduction type, in this case, a
p-type silicon (hereinafter referred to simply as Si), having a specific
resistivity of 10 ohm.cm and, then, an SiO.sub.2 film of 20 nm thickness
is formed as a gate insulator 102' for a MOS type field effect transistor.
Then, a polycrystalline Si of 200 nm thickness as a gate electrode is
formed and patterned by known lithography and etching so that wordlines
103 for a d-RAM memory cell comprising 1-transistor and 1-capacitor are
formed.
The wordlines 103 are covered in a self-aligned manner with an insulator.
For this purpose, an SiO.sub.2 film 106 is deposited to a thickness of 300
nm on the polycrystalline Si, then a resist pattern is formed by
lithography on the SiO.sub.2 film so as to form a mask for etching, the
SiO.sub.2 film is etched and then the polycrystalline Si is etched by
using the SiO.sub.2 film as a mask to form the wordlines 103.
Then, an SiO.sub.2 film was deposited again over the entire surface to a
thickness of 100 nm and then applied with anisotropic dry etching to leave
an SiO.sub.2 film 106' on the side walls of the wordlines 103.
Subsequently, an n type impurity layer 104 as a source and drain of a MOS
type field effect transistor is formed by carrying out As ion implantation
under the conditions at an acceleration voltage of 50 kV and at a dose of
2.times.10.sup.15 cm.sup.-2 and applying heat treatment at 900.degree. C.
for 10 min.
Then, an SiO.sub.2 film is deposited again to a thickness of 70 nm, a
resist mask having an opening is formed by using lithography and the
SiO.sub.2 film is etched by using the resist mask to form an opening 105.
In this state, the wordlines 103 are covered with SiO.sub.2 films 106, 106'
and the surface has a hill and valley structure as shown in FIG. 3, with
the height of the step as high as about 500 nm.
Then, data lines are formed on them.
At first, polycrystalline Si is deposited to a thickness of 700 nm by a CVD
process while doping phosphorus (P).
Then, it is etched for 600 nm of thickness by isotropic dry etching to form
a polycrystalline Si for the first conductor film 107 of the data line.
With this etching, the first conductor film 107 as the buried conductor is
buried in the opening 105, between the wordlines, etc. to planarize the
surface.
Then, the second conductor film 108 comprising tungsten silicide
(WSi.sub.2) of 150 nm thickness is deposited on the first conductor film
107 by a sputtering process.
An SiO.sub.2 film is deposited to 200 nm thickness over the WSi.sub.2
conductor, on which a data line pattern is formed as a resist by
lithography and the SiO.sub.2 film, the second conductor film 108 and the
first conductor film 107 are successively etched by using the resist as a
mask to form the datalines. The datalines and the wordlines 103 are in a
perpendicular arrangement with respect to each other, as shown in FIG. 3.
Subsequently, an SiO.sub.2 film is deposited over the entire surface and a
d-RAM having a lamination capacitance type d-RAM memory cell is fabricated
by way of the steps such as providing an opening for attaining a
connection between a charge storage electrode (not illustrated) and the
n-type impurity layer of the Si substrate 101 of the memory cell,
formation of the charge storage electrode (not illustrated), formation of
a capacitor insulator (not illustrated), formation of a plate electrode of
the capacitor (not illustrated), formation of a first passivation film
(not illustrated), formation of a contact hole, formation of wirings, etc.
With the procedures as described above, a semiconductor device realized is
comprised of a semiconductor substrate, a semiconductor element disposed
on the semiconductor substrate and wirings disposed on a structure having
hills in an underlying material and connected with the semiconductor
element (for example, from the formation of the underlying layers),
wherein the wirings comprise at least two layers of conductor films having
a first conductor film as a lower layer and a second conductor film as an
upper layer, in which the first conductor film having a surface moderating
or planarizing the hills and valleys in the underlying material is
obtained.
Then, FIG. 4 shows a cross sectional view in the course of fabricating the
semiconductor device by the fabrication method as discussed by the present
inventors earlier in this application.
After conducting the steps as far as the formation of the data lines in the
same procedures as described above, a P-doped polycrystalline Si 301 of
100 nm thickness is formed, on which a WSi.sub.2 film 302 of 150 nm
thickness is formed by means of a sputtering or CVD process.
In this case, the data lines are formed on the wordlines 103 conforming to
the steps thereof as shown in FIG. 4.
Comparing values standarized with the length and the width of the data
lines when viewed as a plane, for the sheet resistance of the datalines,
the sheet resistance of the data lines in the method shown in FIG. 4 is
about 70 ohm, whereas the sheet resistance of that according to this
example is as low as 15 ohm.
This is less than 1/4 as that obtained by the method shown in FIG. 4.
In FIG. 4, the sheet resistance is large since the actual length of the
dataline is increased as compared with the length when viewed as plane and
since the step coverage of WSi.sub.2 is not good.
The sheet resistance 15 ohm obtained in this embodiment is substantially
the same as the value for a sheet resistance of a stacked film comprising
150 nm of WSi.sub.2 and 100 nm of polycrystalline Si formed in a planar
surface.
By reducing the sheet resistance of the data lines, d-RAM access time can
be shortened by about 30%.
Embodiment 2
The second embodiment of the present invention will be described with
reference to FIG. 5.
In the same procedures as those in the first embodiment, polycrystalline Si
as a gate electrode is covered in a self-aligned manner to form an n-type
impurity layer for source and drain (not illustrated).
Then, an SiO.sub.2 film of 100 nm thickness is deposited to form an opening
203 above the polycrystalline Si 202 in the wiring portion above an
isolation region 102 of a gate electrode.
Then, a polycrystalline Si is deposited to 700 nm thickness by a CVD
process and the polycrystalline Si is etched for the thickness of 700 nm
by means of isotropic dry etching.
Then, polycrystalline Si is again deposited to 100 nm thickness by a CVD
process, P is ion implanted under the condition at an acceleration voltage
of 50 kV and at a dose rate of 5.times.10.sup.15 cm.sup.-2 and, further,
the polycrystalline Si is activated by an annealing at 900.degree. C. for
30 min to obtain a first conductor film 107.
Then, after depositing a second conductor film 108 made of WSi.sub.2 to 150
nm thickness by a sputtering process, a resist pattern is formed by
lithography and a second conductor film 108 and the first conductor film
107 are anisotropically dry etched by using the pattern as the mask. The
steps are shown in FIG. 5.
FIG. 6 shows a structure as discussed by the present inventors.
As shown in FIG. 6, after forming an opening 203, a polycrystalline Si 402
is formed to 100 nm thickness thickness and activated, WSi.sub.2 403 is
deposited to 150 nm by means of a sputtering process, a resist pattern is
formed by a lithography and WSi.sub.2 403 and the polycrystalline Si 402
are anisotropically dry etched by using the pattern as a mask.
In this case, WSi.sub.2 is deposited in a overhang shape at the step of the
gate polycrystalline Si 404.
Accordingly, when WSi.sub.2 is anisotropically dry etched, an etching
residue 405 is formed as shown in FIG. 6 and wirings of WSi.sub.2 and the
polycrystalline Si are short circuited at the step of the gate
polycrystalline Si.
In FIG. 5 using the fabrication method according to the present invention,
the second conductor film 108 does not form an overhang at the step of the
gate polycrystalline Si, to cause no short circuitting.
After fabricating wirings comprising a stack of WSi.sub.2 and
polycrystalline Si films, a passivation film is formed, and opening, Al
wiring, etc. are applied thereto, to fabricate an LSI comprising a MOS
type field effect transistor.
According to the application of the present invention, since short-circuit
of wirings no more occur between the WSi.sub.2 and the polycrystalline Si
and fine gaps between the gate polycrystalline Si is planarized, Al
wirings do not cause short circuit to attain a preferred production yield
for LSI.
Embodiment 3
The same procedures as those in Embodiment 1 are conducted as far as the
formation of the datalines and then a polycrystalline Si layer as a first
conductor film is formed to a thickness of 700 nm.
The first conductor film is etched for a portion of 600 nm thickness to
planarize hills and valleys in the underlying material and is further
fabricated into a desired wiring pattern.
Tungsten W is deposited on the first conductor film, then, W on a portion
of the first conductor film is silicided by heating to prepare a second
conductor film.
An unreacted portion is removed to form a wiring pattern of low
resistivity.
Substantially the same effects can also be obtained by using other metals
than W that can be formed into silicides such as Ti, Mo, Ta, Pt and Co.
Embodiment 4
The same procedures as those in Embodiment 1 are conducted as far as the
formation of the dataline and then the polycrystalline Si layer as a first
conductor film is formed to a thickness of 700 nm.
The first conductor film is etched for a portion of 600 nm thickness to
planarize hills and valleys of underlying material and is further
fabricated into a desired wiring pattern.
W is selectively deposited on the first conductor film by means of a CVD
process to prepare a second conductor film and then formed into a wiring
pattern.
Selective deposition can also be attained by using Al instead of W.
The present invention is not restricted only to the embodiments described
above but various modifications are possible within a range not departing
the gist of the present invention.
For instance, in Embodiments 1 and 2, the material for the first conductor
film is not restricted only to the polycrystalline Si but similar effects
can also be obtained by using W or TiN as the conductive material having
good step coverage by means of a CVD process.
Also, the material for the second conductor film is not restricted only to
WSi.sub.2, but similar effects can also be obtained by using MoSi.sub.2,
W, TiN, Mo or Al.
According to the present invention, since a structure in which a low
resistivity material is deposited on a planarized hill and valley
structure can be obtained, the resistivity of the wiring can be reduced.
This can also increase the operation speed of LSI.
Further, since electric connection with the underlying material is made by
the first conductor film as the lower layer having a good coverage, the
reliability in the electric connection can be increased.
Further, since the thickness of the insulator to be etched upon opening for
electric connection is substantially uniform, etching for the formation of
the opening can be applied with ease.
Furthermore, since the material for the first conductor film of a good step
coverage is isotropically etched to obtain a structure in which steps are
moderated and the second conductor film comprising the low resistivity
material is formed thereover, there is an advantageous effect that the no
overhangs are formed on the surface and there is no etching residue in the
step portion in the wiring fabrication, to eliminate short circuit of
wirings.
Embodiment 5
The fifth embodiment of the present invention will now be described with
reference to FIG. 7.
FIG. 7(b), (c) are cross sectional views for a memory cell region in the
fifth embodiment of a semiconductor memory device according to the present
invention and FIG. 7(a) is a cross sectional view for a peripheral circuit
thereof.
Further, FIG. 7(d) is a view for the arrangement of the device, in which a
cross section taken along line A--A' corresponds to FIG. 7(c) and a cross
section taken along line B--B' corresponds to FIG. 7(b).
In FIG. 7(a) through FIG. 13, are depicted a support substrate,
specifically, a p-type Si substrate 11, 21, an insulation region
specifically, an SiO.sub.2 region 12, 12, a conductor film, specifically,
polycrystalline Si 13, 56, 63, a storage electrode 14, 27, 76, an impurity
region, specifically, an n-diffusion layer 15, 25, a plate electrode 16,
26, 77, an insulator film, specifically, SiO.sub.2 17, 27, 51, 52, 54,
61, 64, 105, 106, a data line 19, 28, 75, a wordline 23, 72, a gate
electrode 41, 101, Si.sub.3 N.sub.4 53, Si.sub.3 N.sub.4 102 as buried
film, a resist 55, a resist pattern 62, 65, active region 71, hole pattern
73, 74, a dummy wordline 81, a capacitor insulator film 103 and a wiring
conductor 104, respectively.
As shown in the figures, in the memory cell region, since gaps between
wordlines (in the subsequent descriptions, "wordline" sometimes means
including also the insulator films disposed at the periphery of the
wordline as well as the wordline per se for the convenience of the
description) are substantially made planar with the insulator films to
planarize the data lines and the resistance and the parasitic capacitance
of the data lines can be reduced simultaneously.
Further, in the memory cell region, the plate electrode 16 of a capacitor
is fabricated and, the wiring conductor 104 is deposited, so that the
interlayer insulators present so far between the plate electrode and the
wiring layer is eliminated.
Accordingly, the step between the memory cell region and the peripheral
circuit region is reduced by so much as the film thickness of SiO.sub.2
(for example, 0.5 .mu.m).
FIG. 8 is a cross sectional view of an element for illustrating fundamental
steps of fabricating a semiconductor memory device according to the
present invention shown in FIG. 7.
As shown in FIG. 8(a), a p-type Si substrate 11 is used and an SiO.sub.2
region 12 for isolation is formed at the surface of the substrate by using
a known isolation fabrication technique.
Successively, a polycrystalline Si 13 of 150 nm thickness and an SiO.sub.2
51 of 200 nm thickness are formed respectively by a chemical vapor
deposition process (CVD process), to form word lines of a laminate
structure.
Further, an SiO.sub.2 105 of 150 nm thickness is deposited by using a CVD
process and fabricated anisotropically to form a side wall spacer of
SiO.sub.2 105 on the side wall of the wordline.
Then, P is ion implanted at an acceleration voltage of 25 KeV and at a dose
rate of 1.times.10.sup.13 /cm.sup.2 to form an n-type diffusion layer 15.
Then, as shown in FIG. 8(b), an Si.sub.3 N.sub.4 102 of is deposited to 400
nm thickness by using a CVD process and etched for a portion of the film
thickness to bury Si.sub.3 N.sub.4 102 as a buried film between the
wordlines.
The Si.sub.3 N.sub.4 102 at a portion in which the n-type diffusion layer
15 on the surface of the substrate is to be in contact with the data line
is etched by using a resist pattern (not illustrated).
Successively, a polycrystalline Si of containing n-type impurity is
deposited to 200 nm thickness to form a data line 19 by using a CVD
process and, further, an SiO.sub.2 17 is deposited to 200 nm thickness by
using a CVD process (FIG. 8(c)).
As the data line, a low-resistivity high melting metal silicide such as
WSi.sub.2, TiSi.sub.2 or PtSi.sub.2 or a laminate film thereof may be used
instead of the polycrystalline Si.
The SiO.sub.2 17 and the data line 19 are anisotropically fabricated by
using a resist pattern (not illustrated) to form the data lines of a
desired pattern.
Then, an SiO.sub.2 52 is deposited to 150 nm thickness by using a CVD
process and then etched by anisotropic dry etching to form an SiO.sub.2
side wall spacer on the side wall of the dataline to insulate the dataline
(FIG. 8(d)).
An Si.sub.3 N.sub.4 13 of 50 nm thickness and an SiO.sub.2 54 of 300 nm
thickness are, respectively, deposited by means of a CVD process (FIG.
8(e)).
After anisotropically etching the SiO.sub.2 54, Si.sub.3 N.sub.4 53 and
Si.sub.3 N.sub.4 102 by using a resist pattern not illustrated, a
polycrystalline Si 56 containing an n-type impurity is deposited to 50 nm
thickness by a CVD process.
A resist 55 is buried within a recess (FIG. 8(f)).
The polycrystalline Si 56 is not covered with the resist 55 is
anisotropically etched, then the resist 55 is eliminated and, further, the
SiO.sub.2 54 on the memory cell region is eliminated to form the
polycrystalline Si 56 as a storage electrode 14.
Subsequently, Ta.sub.2 O.sub.5 is deposited to form a capacitor insulator
film 103.
For the capacitor film 103, Si.sub.3 N.sub.4, SiO.sub.2, a ferroelectric
material or a composite film thereof can be used.
Successively, W is deposited to form a plate electrode 16.
For the plate electrode 16, low resistive conductor such as high melting
metal, high melting metal silicide, TiN, Al or Cu other than the
polycrystalline Si and W can be used.
Further, a wiring metal 104 is laminated on the plate electrode (FIG.
8(g)).
In the fabrication steps from FIGS. 8(a) to (g) described above, the
following structure shown in FIG. 8(h) to (l) are formed in the region
other than the memory cell region.
In a case where the structure shown in FIG. 8(e), (f) is formed in the
memory cell region, a structure shown in FIG. 8(h), (i) is formed
correspondinly in the peripheral circuit region.
That is, an Si.sub.3 N.sub.4 53, and SiO.sub.2 54 and a polycrystalline Si
56 are formed on a gate electrode 101.
Then, during formation of the structure shown in FIG. 8(g) in the memory
cell region, a structure shown in FIG. 8(j), (k) and (l) is formed in the
peripheral circuit region.
In a state where the storage electrode is formed on the memory cell region
shown in FIG. 8(g), the polycrystalline Si 56 has been etched and no more
present in the peripheral circuit region as shown in FIG. 8(j).
After depositing a capacitor insulator film 103 and a plate electrode
material 16 (FIG. 8(k)), the plate electrode 16 is etched and,
successively, a contact hole 57 is opened and a wiring material 104 is
deposited (FIG. 8(l)).
By using the steps as described above, a semiconductor memory device having
a structure equivalent to that shown in FIG. 7 can be formed.
The embodiment to be described next shows a fabrication method avoiding the
difficulty for fabricating an extremely thick Si.sub.3 N.sub.4 film
(Si.sub.3 N.sub.4 102 in FIG. 8(e), (f)) upon forming the storage
electrode.
After forming the structure in FIG. 8(a), an SiO.sub.2 61 of 50 nm
thickness and an Si.sub.3 N.sub.4 102 of 400 nm thickness are respectively
deposited by a CVD process, and the Si.sub.3 N.sub.4 102 is etched by the
thickness of the film to form a structure shown in FIG. 9(a).
The Si.sub.3 N.sub.4 102 and the SiO.sub.2 61 are anisotropically etched
from a portion in which the n-type diffusion layer on the surface of the
substrate and the data lines are to be connected electrically and from a
portion in which the n-type diffusion layer on the surface of the
substrate and the storage electrode are to be electrically connected by
using a resist pattern 62 (FIG. 9(b)).
A polycrystalline Si containing n-type impurity is deposited to 400 nm
thickness by using a CVD process, and etched substantially for the film
thickness and a polycrystalline Si 63 is buried to the inside of a hole
formed by the etching.
Then, after depositing an SiO.sub.2 64 to 50 nm thickness by a CVD process,
only the portion of the SiO.sub.2 on a region in which the n-type
diffusion layer on the surface of the substrate and the data line are to
be connected is etched by using a resist pattern 65 (FIG. 9(c)).
In this case, although a known etching back method is used for burying the
polycrystalline Si 63, a selective CVD process or a selective epitaxial
growing process (a method of selectively depositing Si only to the portion
in which the Si surface is exposed) capable of obtaining a similar
structure may also be used.
Subsequently, by using substantially the same fabrication method as those
in FIG. 8(c) through (g), a memory cell equivalent with that in FIG. 7 can
be formed.
In this example, since a structure in which the polycrystalline Si 63 is
buried in a desired region between the wordlines is used, thick Si.sub.3
N.sub.4 is no more present between the wordlines and, accordingly, this
can avoid the difficulty of fabricating an extremely thick Si.sub.3
N.sub.4 film upon forming the storage electrode.
In each portion in FIG. 8 and FIG. 9 described above, two kinds of
insulator films SiO.sub.2 and Si.sub.3 N.sub.4 are used selectively.
The purpose of such selective use is for etching the insulator films at
different etching rates, that is, for applying so-called selective
etching.
Similar effects can also be obtained by replacing the materials for the two
kind of films with each other.
In the example described above, explanations have been made to an example
of using the electrode structure shown in the drawings with an aim of
increasing the surface area of the storage electrode, but it will be
apparent that the present invention is applicable also to a structure in
which thin storage electrodes are stacked in a plurality of layers or a
structure of having a storage electrode irrespective of the thickness.
Then, embodiments for the layout of the semiconductor memory device
according to the present invention will be explained referring to FIG. 10.
FIG. 10 shows a system used generally at present for d-RAM, in which only
one of two intersections formed between wordlines and data lines actually
constitute a memory cell, that is, so-called 2-intersection/data system
layout.
FIG. 10(a) shows an embodiment of a layout in which an active region 71 is
made orthogonal to wordlines 72 and the datalines 75.
The layout shown in the figure is applicable to any of two kinds of
fabrication methods shown in FIG. 8 and FIG. 9.
In the figure, 73 denotes a hole pattern formed by etching the Si.sub.3
N.sub.4 102 and the SiO.sub.2 61 in the steps of FIG. 9(b).
74 denotes a hole pattern formed by etching the Si.sub.3 N.sub.4 102 in a
connection region between the data line 19 and the n-type diffusion layer
15 shown in FIG. 8(c), and a hole pattern formed by etching the SiO.sub.2
64 in FIG. 9(c).
76 denotes a storage electrode and 77 denotes a plate electrode.
FIG. 10(b) shows an embodiment of a layout different from that shown in
FIG. 10(a).
The fabrication steps for them are identical.
The layouts for both of them are different with respect to the shape of the
active region 71.
In FIG. 10(b), the layout is different from that of FIG. 10(a) in that the
active region 71 is substantially in parallel with the data line.
FIG. 11 shows a layout in which an intersection between a wordline and a
dataline constitutes a memory cell, that is, so-called, intersection/data
system layout.
The structures shown in FIG. 11(a) and (b) are identical with each other
excepting that the structure of the active region 71 is different.
In these examples, a dummy wordline 81 is used mainly for keeping the
wordline spaces substantially uniform in the memory cell region.
However, in FIG. 11(b), the dummy wordline 81 is used also as an isolation
gate between adjacent MOS FET (metal oxide semiconductor field effect
transistor).
That is, in FIG. 11(b), a MOS FET is constituted with the dummy wordline 71
and the active region 71 therebelow and, if the potential for the dummy
wordline is set to less than the threashold voltage for the MOS FET, MOS
FET can be cut off to prevent leak current from flowing between adjacent
MOS FET.
Although the layouts shown FIG. 11(a) and (b) are different from the
embodiment shown in FIG. 10, the fabrication method is identical with that
of FIG. 10.
Layouts shown in FIG. 12(a), (b), (c) and (d) are, respectively,
modifications for the layouts of the plate electrode 77 shown in FIG.
10(a), (b) and FIG., 11(a), (b).
Each of them has a structure in which the plate electrode is isolated in
parallel with the wordline.
In these embodiments, the plate electrode has a structure in which the
plate electrode material and the wiring material are laminated.
The structure of isolating the plate electrode is a system proposed in 1989
IEEE International Solid-State Circuits Conference, pp 238-239, that is, a
structure capable of attaining a high S/N ratio (high signal-to-noise
ratio (SNR)).
For attaining SNR, it is necessary to drive the potential of the divided
plates at high speed.
Since the memory device according to the present invention has a structure
in which the low resistivity layer used as the wirings in the peripheral
circuits is laminated on the plate electrode, the wiring resistivity of
the plate is low and it is suitable to high speed operation.
The layouts shown in FIG. 13(a), (b), (c), (d) provide respectively,
modified layouts for the plate electrode 77 shown in FIG. 10(a), (b) and
FIG. 11(a), (b).
The four embodiments shown in FIG. 13 are different from the four
embodiments in FIG. 12 in view of the layout of the plate electrode and
have a structure in which the plate electrode is isolated in parallel with
the data lines.
The effect is similar to that of the four embodiments in FIG. 2 and high
S/N ratio can be attained.
In the semiconductor memory device by the fifth embodiment according to the
present invention, the effective length of the data line is reduced to
about 60% of that in the conventional semiconductor memory device by
planarizing the data line and, accordingly, the resistivity and the
parasitic capacitance of the data line can be reduced to about 60% of them
in the conventional device.
Further, as a result of using a structure in which the wiring layer is
directly deposited on the plate electrode, the plate electrode and the
wiring layer can be formed substantially with an identical layer.
Accordingly, the insulator films present so far between the plate electrode
and the wiring layer is saved and the step can be reduced by so much as
the thickness thereof.
Accordingly, the step can be moderated by about 0.5 .mu.m in the peripheral
circuits.
Embodiment 6
In this embodiment, descriptions will be made to a method of forming
wirings as one of the features of the present invention with reference to
FIGS. 14 and 15.
In FIGS. 14 through 19, are depicted an Si substrate 1, 101, a gate
insulator film 2, 102, wordlines 3, 103, an insulator film 4, 5, 104, a
polycrystalline Si 105, an opening 203, a metal silicide 8, 106, a step 9,
an impurity region 10, a contact portion 206 of the data line, a silicon
oxide film 209, a polyimide resin 211, an opening 212, a contact hole 213
disposed to the opening, a polycrystalline silicon 214 as a conductor film
and a conductor film 7, 207, 406 as a buried film, respectively.
FIG. 14 shows a method as discussed by the present inventor.
The SiO.sub.2 102 is formed to 20 nm thickness by thermal oxidation process
on the Si substrate 101 and then the polycrystalline Si 103 is deposited
to 350 nm over the entire surface by a vapor deposition process.
Further, the SiO.sub.2 104 is deposited to 100 nm thickness by a vapor
phase deposition process.
The SiO.sub.2 104 and the polycrystalline Si 103 are fabricated by a
well-known lithography and dry etching to form a periodical wiring pattern
corresponding to the d-RAM word lines.
The width of the wirings is set to 0.5 .mu.m while the wiring gap is set to
1 .mu.m.
Then, after covering the exposed side wall of the polycrystalline Si 103
with the SiO.sub.2 104, the polycrystalline Si 105 and the metal silicide
106 corresponding to the data line are formed.
At first, the polycrystalline Si 105 is deposited to 20 nm thickness by a
vapor phase deposition process.
Then, the metal silicide 106 is deposited to 150 nm thickness by a
sputtering process.
The state is shown in FIG. 14(a).
In the wiring structure in which the metal silicide 106 is deposited on the
polycrystalline Si 105, the wiring resistance can be lowered by about one
digit as compared with the case of using the wiring Si 105 alone.
This is because the resistivity of the metal silicide is as low as about
1.times.10.sup.-4 ohm.cm as whereas the resistivity of the polycrystalline
Si is about 1.times.10.sup.-3 ohm.cm.
Accordingly, the actual wiring resistivity depends on the resistivity of
the metal silicide layer 106.
In this discussed case, a d-RAM, for instance, can be applied sufficiently
to a 4 MB LSI, but application to an LSI of higher integration degree is
difficult, because the wiring resistivity is increased due to the increase
of the wiring length.
Another problem in this discussed case is that the metal silicide 106 is
liable to remain in the step upon fabricating the laminate film into
wirings as shown in FIG. 14(b) and, accordingly, the polycrystalline Si
105 therebelow remains unetched tending to cause inter-wiring
short-circuit.
An embodiment that embodies one of the features of the present invention
will be explained with reference to FIG. 15.
An SiO.sub.2 2 of 20 nm thickness is formed by a thermal oxidation process
on the surface of an Si substrate 1, a polycrystalline Si 3 of 350 nm
thickness is deposited thereover by a vapor deposition process and,
further, an SiO.sub.2 4 of 200 nm thickness is deposited by a vapor
deposition process.
Then, periodical wirings corresponding to wordlines are fabricated by
lithography and dry etching and, further, an SiO.sub.2 5 of 100 nm
thickness is deposited by a vapor deposition process.
Since it is necessary that the SiO.sub.2 5 be deposited so as not to form
overhangs at the stepped portion, SiH.sub.4 (monosilane) and N.sub.2 O
(dinitrogen monoxide) are used within a temperature range of 750.degree.
C. to 800.degree. C. as the conditions for the vapor deposition (the
conditions are used for all of SiO.sub.2 vapor deposition described
subsequently).
As a result, a groove 6 having a vertical side wall is formed (FIG. 15(a)).
The procedures so far are the same as those in the case discussed above.
Then, a polycrystalline Si 7 as a buried film is deposited to a thickness
sufficiently greater than one-half of the width of the groove 6 by a vapor
deposition process so as to completely fill the groove 6 (FIG. 15(b)).
Then, the polycrystalline Si 7 is etched back for the entire surface to
reduce the entire thickness and planarize the surface.
The etching back used herein may be a wet process etching, but a dry
etching process using a halogen gas provides good controllability (FIG.
15(c)).
Subsequently, a tungsten silicide (WSi) 8 of 150 nm thickness is deposited
as a metal silicide by a sputtering process (FIG. 15(d)).
In this embodiment, the step present in the underlying material can be
eliminated to attain planarization by etching back the polycrystalline Si
formed as a thick layer and, as a result, WSi can be formed as
substantially planar wirings.
Accordingly, as compared with the discussed case shown in FIG. 14 in which
wirings are formed just conforming with the steps of the underlying
material, the wiring length can be shortened by so much as the steps are
eliminated and there is obtainable an effect of lowering the wiring
resistivity.
Further, since the WSi is formed planar, fabrication becomes facilitated to
eliminate the occurrence of etching residue.
Further, in an actual LSI pattern, terminal ends of the wirings disposed
periodically are always present to cause a step 9 as shown in FIG. 15(e).
However, also in this case, the slope of the step is remarkably moderated
by etching back the polycrystalline Si and the metal silicide can be
fabricated without causing etching residue.
Further as shown in FIG. 15(e), there is a case of forming a connection
with an impurity diffusion layer 10 formed on the surface of the
semiconductor substrate. Also in this case, since the inside of the groove
is completely buried with the polycrystalline Si, connection between the
metal silicide and the impurity diffusion layer can be ensured easily.
One of the features of the present invention is to use the polycrystalline
Si formed by introducing impurities upon film formation.
Table 1 shows a result of measuring and comparing the wiring resistivity
between the discussed case and the examples of the present invention.
TABLE 1
______________________________________
Present invention
Discussed case
WSi/polycrystalline
Polycrystal-
WSi/poly
Structure
Si line Si alone
crystalline Si
______________________________________
Wiring 15 ohm/ 120 ohm/ 54 ohm/
resistivity
Sheet 14 ohm/ 71 ohm/ 14 ohm/
resistance
in the
planar
portion
______________________________________
As apparent from the result above, even in wirings disposed across the
steps on the underlying material the present invention can provide an
effect of attaining a resistance equal with that in a case free from steps
in the underlying material (Table 1: sheet resistance).
Further, the result above shows that the resistance value of the laminate
wiring comprising WSi/polycrystalline Si is controlled by WSi.
Accordingly, one of the features of the present invention is to form
wirings by laminating a titanium silicide or tungsten of lower resistivity
than WSi on a planarized polycrystalline Si.
Embodiment 7
In this embodiment, descriptions will be made to a method of fabricating a
d-RAM memory cell by using a wiring forming method as described in
Embodiment 6 and further simplifying the fabrication steps by using an
organic material with reference to FIGS. 16 and 17.
At first, FIG. 16 shows an embodiment of a plane layout of a d-RAM memory
cell.
Wordlines 1, 2, 3, 4 disposed vertically and data lines 1, 2, 3, 4 disposed
laterally intersect with each other.
For better understanding of the drawings, boundaries for the wirings of the
wordline 2 and dataline 3 are depicted by hatched lines.
In the figure, the wordline 2 and the data line 3 are to be noted in a
sense of selecting one of memory cells.
When the wordline 2 is turned on, charges stored in a capacitor forming
region D flow by way of a contact B, a semiconductor substrate just below
the wordline, and the data line contact C to the data line 3 to transmit
information.
An embodiment of the present invention will be explained with reference to
FIG. 17 showing a cross sectional shape at a flexed broken line taken
along A--A'.
An SiO.sub.2 202 of 500 nm thickness as an isolation region is formed on a
p-type Si single crystal substrate 201 of 10 ohm.cm by using a well-known
process.
An SiO.sub.2 203 of 10 nm thickness as a gate oxide film of a MOS
transistor is formed by a thermal oxidation process, a polycrystalline Si
of 350 nm thickness is deposited by a vapor deposition process and,
further, an SiO.sub.2 205 of 250 nm thickness is deposited by a vapor
deposition process.
An SiO.sub.2 205 and a polycrystalline Si are fabricated by lithography and
dry etching to form a wordline 204.
Then, after covering the side wall of the wordline 205 with the SiO.sub.2
205, the SiO.sub.2 in the data line contact region 206 (corresponding to
the region C in FIG. 16) is eliminated by using lithography and dry
etching (FIG. 17(a)).
Then, the data lines are formed by the method as described in Embodiment 6.
At first, a polycrystalline Si 207 of 500 nm thickness is deposited by a
vapor deposition process.
The polycrystalline Si 207 is deposited while introducing and incorporating
a phosphine (PH.sub.3) gas into the film in the course of deposition.
In this embodiment, since the gap between the wordlines 204 is set to 300
nm, the groove formed between the wordlines can be filled completely by
setting the film thickness of the polycrystalline Si to 500 nm.
As also described previously with respect to Embodiment 6, one of the
features of the present invention is to set the film thickness of the
polycrystalline Si such that the recesses present in the underlying
material are buried (FIG. 17(b)).
Subsequently, the polycrystalline Si 207 is etched back by a dry etching
process using a halogen gas of sulfur hexafluoride (SF.sub.6).
It is controlled such that the polycrystalline Si 207 remains by about 100
nm on the wordline.
The surface of the polycrystalline Si is substantially planar even just
after the formation. However, if the metal silicide is laminated directly
to form the data line, the step is increased due to the fabrication of the
data line and the data line itself to bring about a problem, for example,
making it difficult for the fabrication in the subsequent wiring steps. It
is, accordingly, one of the features of the present invention to reduce
the substantial film thickness by etching back the polycrystalline Si and
reduce the step in the data line itself.
Referring to the range of the etching back, it is desirably to eliminate a
portion for 80.+-.10% of the thickness of the thus formed film (FIG.
17(c)).
Then, a WSi 208 of 150 nm thickness is deposited by using a sputtering
process (FIG. 17(d)).
Further, an SiO.sub.2 209 of 300 nm thickness is deposited by a vapor
deposition process.
The SiO.sub.2 209, the WSi 208 and the polycrystalline Si 207 are
fabricated by lithography and dry etching to form bit wirings (FIG.
17(e)).
An SiO.sub.2 210 of 100 nm thickness is formed over the entire surface by a
vapor deposition process to cover the side walls for the WSi 208 and the
polycrystalline Si 207 (FIG. 17(f)).
Then, a capacitor is formed.
A polyimide resin 211 is formed to a thickness of 700 nm in a planar
portion by a spin coating method.
As the polyimide resin, type PiX-L110 manufactured by Hitachi Kasei, for
example, can be used.
After coating the polyimide resin, a heat treatment is applied in an
atmosphere of lower than 1 atm. at 600.degree. C. for 22 min.
Then, a hole 212 is formed in a capacitor forming region by the multi-layer
resist method as described previously in Embodiment 3.
The polyimide resin can be fabricated only with active oxygen like that
photoresist, etc. (FIG. 17(g)).
Then, only the SiO.sub.2 210 on the Si substrate 201 is selectively removed
by using the polyimide resin 211 as a mask, to expose the surface of the
Si substrate and form a hole contact 213.
For the selective elimination of the SiO.sub.2 210, dry etching process by
a halogen gas is used (FIG. 17(h)).
Since an extremely thin denatured layer may sometimes be formed on the
surface of the polyimide upon dry etching of SiO.sub.2, it is desirable in
such a case to slightly etching the surface by using oxygen.
Then, a polycrystalline Si 214 of 100 nm thickness as a storage electrode
for the capacitor is formed by deposition while not removing but leaving
the polyimide resin that was used as the mask for the formation of the
hole contact 213.
Impurities may be introduced to the polycrystalline 214 by incorporating
them at the same time with the film formation like that in the
polycrystalline Si 207, or by ion implantation after the formation of the
film.
In the case of using ion implantation, however, it is necessary to implant
the ions such that they are entered orthogonally, since the
polycrystalline Si stands vertically on the side wall of the polyimide
resin (FIG. 17(i)).
Then, a photoresist is formed over the entire surface to a 1 .mu.m
thickness in the planar portion by a spin coating method and etched back
to leave the photoresist 215 only in the hole and the surface of the
polycrystalline Si 214 on the polyimide resin 211 is exposed (FIG. 17(j)).
The polycrystalline Si 214 exposed on the polyimide resin 211 is
selectively eliminated by a dry etching process (FIG. 17(k)).
The polyimide resin 211 and the photoresist 215 are eliminated by oxygen
plasmas to form shields of the polycrystalline Si as a storage electrode
216 for a capacitor (FIG. 17(l)).
The storage electrode 216, in a plan view observed from the side of the
surface, corresponds to the capacitor forming region D in FIG. 16.
Then, a capacitor insulator film 217 is formed and, further, a capacitor
plate electrode 218 is formed to complete the fabrication the steps for
the memory cell (FIG. 17(m)).
Subsequently, the wiring formation steps are carried out to conduct
connection with peripheral circuits to fabricate an LSI.
In this embodiment, descriptions have been made to the procedures of
fabricating the polyimide resin and, thereafter, the underlying SiO.sub.2
is fabricated to form a hole contact, shields of the polycrystalline Si
with higher accuracy can be formed by forming the hole contact previously
then coating the polyimide in a state of exposing the surface of the Si
substrate and then applying opening fabrication.
In this embodiment, the hole can be formed extremely easily by using, as a
base material for forming shields of the storage electrode, an organic
material capable of providing a selectivity ratio almost infinitely
greater than that of inorganic materials such as SiO.sub.2 or Si.
Accordingly, there is no more requirement for raising the exposed surface
of the Si substrate for making a contact with the storage electrode by
means of the selective CVD process as in the case shown in FIG. 2
discussed previously.
As a result, it is possible to avoid a problem in a case of using WSi to
the dataline that Si nuclei are grown also to the surface of SiO.sub.2,
which should not be grown upon selective CVD fabrication of the
polycrystalline Si due the contamination by the WSi itself, to remarkably
lower the selectivity, and there is obtained an effect capable of
remarkably improving the production yield.
Further, it can also provide an effect of simplifying the fabrication steps
since it no more requires such steps as formation of the polycrystalline
Si by the selective CVD process, formation of Si the nitride film and
formation of the SiO.sub.2. In addition, since the resin can be formed
merely by using a convenient spin coater, it can also provide advantageous
effect on the improvement of the operation safety and reduction of the
cost. Embodiment 8
In this embodiment, descriptions will be made to an application example of
the present invention, in combination with the lithographic technic with
reference to FIG. 18.
FIG. 18 shows a case of using a multi-layered photoresist method.
As already explained in Embodiments 6 and 7, wirings of polycrystalline Si
302 corresponding to d-RAM wordlines are formed on an Si substrate 301 and
the entire surface is covered with an SiO.sub.2 303 formed by a vapor
deposition process to form a pseude step on the Si substrate.
The height of the step is adjusted to about 500 nm.
Then, the the multi-layered resist step is applied.
At first, an organic material 304 as the lower resist material of 1.5 .mu.m
thickness on the planar portion is formed by a spin coating method.
In this case, it is important to select the film thickness of the organic
material 304 such that the surface of the material 304 is made planar.
Then, a SOG (spin coated glass) 305 of 100 nm thickness is formed as an
intermediate layer and, further, a photoresist 306 of 600 nm thickness is
formed as an upper layer resist by a spin coating method.
Then, a pattern is formed to the photoresist 306 by a well-known
photoetching method and the SOG 305 is dry etched by using the photoresist
306 as a mask to transfer the pattern (FIG. 18(a)).
Then, the organic material 304 is etched by well-known dry etching using
oxygen and, further, pattern transfer is conducted.
In this case, the photoresist 306 of the upper layer is eliminated
simultaneously by etching but the SOG 305 is left not being etched with
oxygen and, accordingly, the organic material 304 can be fabricated at a
good accuracy by using the SOG 305 as a mask (FIG. 18(b)).
Then, the SiO.sub.2 302 exposed on the underlying material is selectively
etched by using the organic material 304 as a mask to expose the surface
of the Si substrate and form a contact 307 (FIG. 18(c)).
In the prior art method, it requires complicated steps of once removing the
organic material 304 used for the fabrication of the SiO.sub.2 302,
forming another inorganic material such as silicon nitride over the entire
surface and applying opening or fabrication to the separate inorganic
material by using the multi-layered resist method again.
One of the features of the present invention is not to remove the organic
material 304 used as the mask for the fabrication of the SiO.sub.2 302 but
to leave it as it is and deposit the Si 308 thereover by the vapor
deposition process.
Then, a shield of the polycrystalline Si 308 is formed in accordanace with
the method as described in Embodiment 7 (FIG. 18 (d)).
In this embodiment, since the SiO.sub.2 is selectively eliminated by using
the organic material as a mask to expose the surfcace of the Si substrate
and, subsequently, the organic material used as the mask is not removed
but is used as a supporting base for forming the polycrystalline Si, this
can provide an effect capable of ensuring the connection between the
polycrystalline Si and the the Si substrate and, at the same time, capable
of forming the shield extremely simply and at a good controllability.
The organic material 304 used in this embodiment may be any of other
organic materials providing that than the polyimide resin that the
material can be etched with oxygen.
Further, although the feature of the present invention have been described
regarding the combination with the multi-layered resist method in this
embodiment, it is also possible to former a pattern directly on the
underlying material and then forming the polycrystalline Si by using a
single-resist layered resist method, that is, photoresist or EB resist.
Embodiment 9
In this embodiment, descriptions will be made to an example of burying an
polycrystalline only in hole on a predetermimed region Si by using a
method of forming the polycrystalline Si on an organic material with
reference to FIG. 19.
Wirings of a polycrystalline Si 402 are formed on an Si substrate 401 and
the entire surface is covered with an SiO.sub.2 403 to form a pseude steps
(FIG. 19(a)).
A polyimide resin 404 is formed by a spin coating method and the polyimide
resin is selectively eliminated from a predetermined region by the
multi-layered resist method as described in Embodiment 3, to form a
conctact hole 405 (FIG. 19(b)).
Then, a polycrystalline Si 406 is deposited so as to bury the contact holes
405 by a vapor deposition process (FIG. 19(c)).
The polycrystalline Si 406 is etched back to leave the polycrystalline Si
406 only at the inside of the contact hole 405 (FIG. 19(d)).
Then, the polyimide resin 404 is removed by oxygen plasmas (FIG. 19(e)).
This embodiment can provide an effect capable of burying the
polycrystalline Si by leaving it only in the predetermined region without
using the selectively CVD process shown in the discussed case of FIG. 2.
According to the present invention, since the wirings disposed across the
steps of the underlaying material can be formed into a substantially
planar structure, it can provide an effect capable of reducing the
resistance of the wirings themselves.
Further, in a memory LSI in which data lines are disposed across the
wordlines disposed periodically since the wiring resistivity of the data
lines can be reduced by substantially planarizing the structure of the
data lines, it is possible to increase the circuit operation speed and
improve the performance of the memory LSI.
In particular, a remarkable effect is obtainable in a so-called shield data
type d-RAM in which data lines are disposed directly by way of an
insulator film on the wordlines.
Further, since the capacitor constituting the memory cell can be formed by
using an organic material, margin for the process can be improved and the
process can be simplified remarkably. In addition, since the use of the
selective CVD is no more necessary, this enables to avoid a problem that
the metal silicide itself constitutes a contamination source and reduce
the selectivity, and lowering the production yield and, accordingly, can
provide an effect of greatly increasing the production yield.
Further, the material used in the lithographic step can be used as it is
for the organic material, steps can be simplified and the a high accuracy
for the pattern shape can be attained.
It is further understtod by those in the art that the foregoing description
is preferred embodiment of the disclosed device and that various changes
and modifications may be made in the invention without departing from the
spirit and scope thereof.
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